There are several procedures for the production of the anthelmintic tetramisole reported in the literature (Raemaekers et al., J. Med. Chem. 9, 545 (1966), Bakelien et al., Aust. J. Chem. 21 1557 (1968), T. R. Roy, U.S. Pat. No. 3,855,234, M. E. McMenin, U.S. Pat. No. 3,845,070). The Raemaekers method involves a reduction step involving sodium borohydride, an expensive reducing agent. The process in U.S. Pat. No. 3,855,234 suffers from lack of regioselectivity and also from problems of isolation of intermediates.
A method for making tetramisole from 1-(2-hydroxyethyl)-4-phenyl-4-imidazolin-2-thione and thionyl chloride is disclosed in U.S. Pat. No. 3,726,894. This compound is made by the hydroboration reaction on 1-vinyl-4-phenyl-4-imidazolin-2-thione.
The literature also reports two methods for the synthesis of levamisole, the levorotatory isomer of tetramisole, without its resolution (Raemaekers et al., Tetrahedron Letters, 1467 (1967) and P. R. Dick, French Patent 2224-472). However, these procedures are not regioselective in that they would also produce some "isotetramisole", (2,3,5,6-tetrahydro-4-phenylimidazo(2,1-b)thiazole), which would have to be separated. No mention of this separation has been made in the above publication.
The instant invention overcomes all these drawbacks in that it is regioselective (no isotetramisole can be formed); all the steps occur in good yields and the intermediates are easy to isolate; it involves for the first time in tetramisole synthesis a catalytic reduction step; and it is applicable to the synthesis of other analogs.